Apparatus and a method for controlling the alkalinity and pH of an industrial process

ABSTRACT

The present invention relates to an apparatus and a method for controlling the alkalinity and the pH of an industrial process. The apparatus is adapted for being located outside a main process stream of the industrial process. The apparatus includes inlet means ( 2, 3, 4 ) for a basic substance, an acidic substance and water, control means ( 6 ) for said inlet means ( 2, 3, 4 ), a reactor ( 1 ) for reacting the basic substance, the acidic substance and the water for providing a predefined alkalinity and pH in the resulting aqueous buffering mixture, and outlet means ( 9 ) for supplying said buffering mixture to the main process stream for controlling the alkalinity and the pH of the industrial process.

The present invention relates to an apparatus and a method forcontrolling the alkalinity and pH of an industrial process.

Alkalinity is a measure of the buffering capacity of an aqueous system,i.e. the capacity to prevent pH changes. It can also be seen as acapacity to accept H⁺ or OH⁻ ions. The buffering capacity enables theaddition of acidic and basic substances to an aqueous system withoutsignificant changes in the pH. The amounts of bases and acids that canbe added without affecting the pH depend on the strength and amount ofthe base and the acid as well as on the amount of alkalinity. Manyindustrial processes lack a sufficient alkalinity required for operatingproperly.

A high alkalinity prevents harmful pH variations caused by addition ofraw materials having different pH values. pH changes may cause e.g.unwanted precipitations and dissolutions in an industrial process.

Generally alkalinity results from the amounts of hydroxide, carbonateand bicarbonate ions in an aqueous solution according to the equation:[alk]=[OH⁻]+2[CO₃ ²⁻]+[HCO³⁻]−[H⁺]. Alkalinity may also result fromammonia and the conjugate bases of phosphoric, silicic, boric andorganic acids.

Alkalinity may be expressed as M-alkalinity or P-alkalinity depending onthe determination used for the assay. M-alkalinity is determined bytitration with acid to the methyl orange endpoint (pH 4.5) andM-alkalinity results mainly from the concentration of bicarbonate ions.P-alkalinity is determined by titration with acid to the phenolphthaleinendpoint (pH 8.3) and P-alkalinity results from the concentrations ofhydroxide and carbonate ions. Alkalinity is often expressed in units ofmg/l of CaCO₃ or mmol/l of CaCO₃.

Alkalinity is not to be confused with pH, which is a measure of theconcentration of H⁺ ions (pH=−log [H⁺]).

In case the aqueous system of an industrial process does not have asufficient buffering capacity the pH control and adjustment may bedifficult. The amounts of acidic and basic substances which will beadded have to be very carefully measured. Especially if strong acids orbases are used any over-dosage will easily cause too big pH changes.

Adequate alkalinity is especially advantageous e.g. in processes formaking pulp or paper as well as in water treatment processes.

In the production of chemical pulp, a pulp suspension for producingpaper or pulp is generally produced in a so called fiber plant bycooking, bleaching, extracting and/or washing operations, wherein the pHis substantially changed between unit operations and wherein a highalkalinity is generally not desirable. Papermaking pulp may also beproduced from recycled paper. When the pulp has been treated and isready for being processed into sheets of pulp or paper, the pulp entersthe paper mill. Such a pulp is called herein a papermaking pulp todistinguish it from the pulp in the fiber plant. In the subsequenttreatment the papermaking pulp is subjected to dilutions with water andto various fine adjustments with chemicals. Papermaking pulp can also beproduced from chips by refining or grinding. Such a papermaking pulp iscalled mechanical pulp.

A papermaking pulp suspension has a low inherent buffering capacity. Ahigher alkalinity in the pulp suspension would be advantageous sincemany chemical additions and treatments which affect the pH are effectedduring the papermaking process. pH fluctuation on the other hand maycause various problems in the process, like in the refining.

A high alkalinity is advantageous in the papermaking e.g. if thepapermaking pulp is acidic when entering the stock preparation and theshort circulation is run at a neutral or alkaline pH or slightly acidic.The pH is traditionally raised and controlled at need by adding sodiumhydroxide, NaOH. NaOH is, however, a very strong base, which means thatonly small amounts are needed for pH adjustments, the NaOH also has tobe diluted before addition. The paper maker could end up in a situationwith varying pH of the entering pulp, which has a negative impact onpaper quality and paper machine runnability.

A paper making pulp wherein the fibers have been treated and are readyfor being used to produce paper is called stock. Through the stockpreparation and the long and short circulation of the paper plant anumber of paper chemicals and dilution waters are added, some of whichare acidic and therefore decrease the pH of the pulp. The paper makercould therefore end up with a too low pH in the short circulation or inthe stock preparation which may lead to dissolution of calcium carbonateused as a filler and/or pigment and to foaming problems due to adecomposition of dissolving calcium carbonate. One might be once againforced to adjust pH using NaOH. The pH may also change at refining or instorage towers due to microbiological growth.

When pulp is bleached using dithionite, dithionite may cause problemswhen it oxidizes. Sulfuric acid is formed in the paper machinecirculations causing pH to decrease in water and pulp.

In water treatment processes a substantial alkalinity would beadvantageous in processing both fresh water and wastewater. In treatmentof drinking water sufficient alkalinity would assure e.g. that the pHdoes not vary too much in the water distribution system. Too bigfluctuations in pH may cause e.g. corrosion in the pipes because of thevariation in the reduction oxidation condition. The inherent hardness ofnatural waters also causes problems in the processing.

In wastewater treatments the alkalinity and the pH of the water areimportant for biological activity. Sufficient alkalinity would ensurethat there is not too much fluctuation in the pH. The pH should not varytoo much and it should stay suitable for biological activity in orderfor the bioprocess to proceed properly.

In the prior art one way of avoiding too big pH fluctuations inindustrial processes has been to add dissolved sodium bicarbonate,NaHCO₃, to the process. The NaHCO₃ dissociates in the aqueous mediaforming bicarbonate ions, HCO₃ ⁻, which have a buffering capacity andtherefore counteract any pH change. NaHCO₃ is a solid powder, which isgenerally supplied in so called big-bags, and the mill needs space forhandling, equipment for dissolving and tanks for storage. The NaHCO₃ ismessy to work with, when in contact with moisture or water.

According to WO 98/56988 the pH of a pulp suspension in the stockpreparation of a paper machine is stabilized by increasing thebicarbonate alkalinity of the suspension. A combination of sodiumhydroxide and carbon dioxide provides a significant buffering effect inthe pulp. There may, however, appear some difficulties in the additionof the sodium hydroxide and carbon dioxide according to the invention. Alocal high pH caused by the addition of the sodium hydroxide may causeprecipitation of calcium carbonate. A high pH may dissolve extractives,which may then precipitate elsewhere in the process. A high pH may alsocause yellowing of the pulp. It may also be difficult to addsufficiently carbon dioxide to the process.

In the prior art the addition of the basic and acidic substances neededin the industrial process is usually done separately in special dosingpoints. The use of basic and acidic substances often requirespreprocessing, e.g. dissolving and diluting, and special equipment forthat before the substances are fed into the industrial process. Theselection of the dosing points depends on the industrial process and thechemicals in question. The dosing points of e.g. gaseous substances mustbe carefully selected to have a suitable pressure, temperature and pulpconsistency. It may not even be possible to feed gaseous substances inthe needed points of the industrial process, or it may require complexequipment.

The amounts of the basic and acidic substances needed have to bemeasured and determined separately for each industrial process becauseof varying process conditions. In case strong acid and bases are used,the amounts added have to be measured especially carefully in order toavoid too big pH changes caused by over dosages. Diluted substances areeasier to add but dilution itself requires extra work.

It could often be important that acidic or basic chemicals needed in theindustrial process could be added without affecting the pH of thesystem.

Thus, there exists a need for a simple and secure way to add acidic andbasic substances into industrial processes for controlling thealkalinity, pH and/or hardness of a liquid in an industrial process.

An object of the present invention is to provide an apparatus which canbe easily connected to an industrial process in order to add basic andacidic substances into the industrial process for controlling thealkalinity and pH at desired points. Another object of the invention isto provide an apparatus which enables automatic and precise feeding ofthe acidic and basic substances to a liquid of the industrial process.

Another object of the invention is to provide a method of controllingalkalinity and pH of an industrial process.

A further object of the invention is to provide a method of controllingthe hardness of water in an industrial process.

The invention according to the present application is defined in theappended claims, the contents of which are included herein by reference.

Consequently, the present invention relates to an apparatus and a methodfor controlling the alkalinity of an industrial process by feeding andmixing automatically water as well as basic and acidic substance intothe process.

The present invention relates to an apparatus for controlling thealkalinity and the pH of a liquid of an industrial process, wherein theapparatus is adapted for being located outside a main process stream ofan industrial process. The apparatus includes inlet means for a basicsubstance, an acidic substance and water, control means for the inletmeans and a reactor for reacting the basic substance, the acidicsubstance and the water. The reactor provides a predefined alkalinityand pH in the resulting aqueous buffering mixture. The apparatus alsoincludes outlet means for supplying the buffering mixture to the mainprocess stream for controlling the alkalinity and the pH of theindustrial process.

The present invention relates also to a method for controlling thealkalinity of a liquid of an industrial process. The method comprisesthe steps of providing a basic and an acidic substance which substancesin combination are capable of forming buffering ions in an aqueousmedium; introducing controlled feeds of the basic and the acidicsubstances as well as water into the reactor of an apparatus locatedoutside a main process stream of the industrial process; causing thebasic substance, the acidic substance and the water to react so as toprovide a predefined alkalinity in the resulting aqueous bufferingmixture; and supplying the aqueous buffering mixture to the main processstream for controlling the alkalinity of the industrial process.

The apparatus according to the invention enables very accuratealkalinity control. It also enables an accurate pH control. Theinvention further enables controlling of the hardness of any aqueousvehicle of the industrial process. The apparatus is easy to install inany necessary dosing points in an industrial process since there is noneed for feeding acid and base directly to the process. The addition ofacidic and basic substances occurs outside the main process in theinventive apparatus automatically and safely without messy handling orrisky reaction between the substances. The location of the apparatus andthe dosing points of the prepared buffering mixture may be freelydetermined depending on the industrial process in question.

An apparatus according to the present invention can be designed in manyways to best suit the industrial process in question. The structure maybe designed depending on the industrial process where it is used as wellas taking into account the substances used in the apparatus forpreparing the buffering mixture.

The apparatus according to the present invention comprises a reactorwhich may be either a continuous reactor or a batch reactor. Acontinuous reactor enables a continuous control of the alkalinity in theindustrial process. In case changes happen in the alkalinity or processconditions, adjustments may be done automatically and without delay. Abatch type reactor apparatus may be operated when there is need foradjusting the alkalinity e.g. due to special process conditions or rawmaterials. A batch type reactor may also feed the buffering mixture to atank where the buffering mixture is continuously and/or intermittentlyfed to the industrial process.

The invention will now be described in greater detail with reference tothe drawings, wherein:

FIG. 1 discloses a preferred embodiment of an apparatus according to thepresent invention.

FIG. 2 presents a schematic drawing of a papermaking process havinginstalled an apparatus according to the present invention.

In the preferred embodiment the apparatus shown in FIG. 1 has a reactordesigned as a tube reactor 1 having one bend 11. The apparatus comprisesinlets means 2, 3, 4 for the basic and the acidic substance and thewater, respectively. The inlet for the acidic substance comprises asmaller perforated tube 5 located inside the tube reactor 1. Theperforated tube 5 has several inlet openings 7. The inlet means 2, 3, 4have each separate control means 6. The control means 6 are individuallyregulated by a control device (not shown) which is preferably connectedto a computer which regulates the main process. The apparatus comprisesalso a temperature controlling and monitoring means 8, a static mixer 10and means for taking samples 12 as well as outlet means 9.

The length of the tube reactor 1 is adjusted depending on the industrialprocess where it is installed. It is important that the tube reactor 1has a length which is sufficient to allow the reaction between thesubstances to take place under controlled conditions. The number of thebends 11 as well as the shape and the size of the tube reactor 1 isadjusted depending on the length of the reactor 1 and the space in thelocation where it is installed.

The inlet means 2, 3, 4 of the apparatus comprise conventional meansused in industrial processes such as valves. The inlet means for thebasic and/or the acidic substance preferably also includes means (notshown) for diluting and/or dissolving the substance in water to providea controlled aqueous solution of said substance. This means that evenconcentrated acids and bases or solid substances are preferably used inthe apparatus without any preprocessing outside the apparatus. Thedilution of a concentrated substance to an appropriate concentration orthe dissolving of a solid substance preferably occurs in thecorresponding inlet means of the apparatus. In such cases no separatestages for dilution and dissolving are needed.

The inlet means 2, 3 for the basic and acidic substance, respectively,may also comprise an inlet for a gas which is capable of forming a baseor an acid in an aqueous medium. Such an inlet is provided e.g. by theperforated tube 5 in FIG. 1 inside the tube reactor 1. The perforatedtube 5 feeds the acidic substance to the reactor 1 through a largenumber of steps continuously and gradually and assures a safe additionof the acidic substance.

The inlet means 4 for water is an inlet for water to be used in theapparatus. The inlet means for water need not be separately provided,but it may instead be combined with the inlet means for the basic and/oracidic substance. In such a case, the water is fed into the apparatusfrom the same point as the basic or the acidic substance. The apparatusmay also comprise separate vessels for pre-combining said basic and/oracidic substance with water. The obtained diluted mixture is thencombined with the other substance in the reactor.

In a preferred embodiment of the invention the apparatus has several,more preferably two or more inlets for said basic and/or acidicsubstance and/or water. Addition of some acidic substances such ascarbon dioxide through 3 to 10 steps is preferred. In a tube reactortype apparatus the inlets are preferably spaced apart along the tubereactor. This enables addition of the substances so that there is enoughtime for the reaction between the added substances to take place beforemore substance is fed into the reactor.

In the preferred embodiment according to FIG. 1 perforated tube 5, whichis located inside the tube reactor 1, provides several inlets. Severalopenings 7 of the perforated tube 5 assure continuous and gradualaddition of the acidic substance into the reactor 1. The length of theperforated tube 5 in relation to the whole tube reactor 1 as well as thenumber of the openings 7 is adjusted according to the amount and flowrate of the acidic substance. The perforated tube 5 is preferably usedas an inlet for a gaseous substance such as carbon dioxide. The openings7 at the distant end of the tube 5 may be larger than at the inlet endin order to compensate for a pressure drop in the tube.

The basic and acidic substance feeds and the water feed into theapparatus are controlled by the control means 6 for the inlet means 2,3, 4. The control means 6 are indicated by boxes in FIG. 1. The controlmeans 6 comprise means for monitoring and adjusting the flow rate of thebasic and acidic substances needed in the reaction. The flow iscontrolled to ensure accurate dosing of the substances. By controllingthe amount of the basic and acidic substances fed into the reactor thechemical equilibrium of the reaction is controlled accurately and apredefined alkalinity and pH is obtained in the resulting aqueousbuffering mixture. Also the concentrations of the substances needed inthe process are preferably controlled and monitored. This way it ispossible to avoid failures due to false amounts and concentrations inthe apparatus.

The control means 8 indicated by a box in the reactor 1 are designed forcontrolling the reaction conditions and concentration of substances inthe apparatus. The controlling of the apparatus also ensures steady andsafe operation of the apparatus. The control means 8 may comprise anyconventional means for measuring and analyzing the necessary parameterssuch as temperature. The control means 8 may also include alarm systemsfor unusual situations.

The outlet means 9 of the apparatus comprise conventional means used inindustrial processes. The outlet means 9 enable supplying the bufferingmixture prepared in the apparatus to the industrial process.

In order to ensure proper mixing of the substances in the apparatus itadvantageously comprises means 10 for spraying and/or agitation. Themixers are preferably static mixers 10 such as those disclosed inFIG. 1. The mixing in the apparatus is preferably automatic andconstant, although it may be separately controlled and monitored.

The apparatus preferably also comprises means for cooling the walls ofsaid reactor in order to keep substances such as calcium carbonate fromprecipitating on the walls. The controls means of the apparatus may alsocomprise means for removal of precipitated matter.

An alkalinity apparatus according to the present invention is preferablyconnected to a control and monitoring system of the industrial processor of the main process stream. Thus, the functioning of the apparatus iscontrolled and monitored simultaneously with the main process. Thepersonnel running the apparatus according to the present invention doesnot have to monitor the apparatus separately as it functions as a partof the main process. In cases where dilution is provided in theapparatus, basic and acidic substances may be used in the apparatuswithout any preprocessing.

In a preferred embodiment of the invention the apparatus is computercontrolled in order to ensure the operation of the apparatus to takeplace in a controlled manner.

The apparatus according to the invention may be used in any industrialprocess where there exists need for controlling alkalinity. Theapparatus may be designed according to the requirements of eachindustrial process. The apparatus may be used e.g. in various points ina process for making paper or pulp, such as in the stock preparation, inwater circulations, in a filter between a pulp mill and a paper mill orin the broke storage tank. It is also possible to use the apparatusaccording to the invention in a water treatment process. The apparatusis useful both in wastewater and fresh water treatments.

FIG. 2 discloses a papermaking process wherein the apparatus 26according to the present invention is connected to a dilution screw 23after a filter 22. The functioning of the apparatus in this kind of acase is explained in greater detail in Example 1.

A person skilled in the art is able to calculate the amount ofalkalinity needed in an industrial process in order to maintain asufficient buffering capacity during the process. The amount ofalkalinity needed depends e.g. on the amount and type of chemicals fedinto the industrial process, raw material flows, filler and pulp amount.

The term “main process stream” as used in the present specification andclaims basically means the flow of combined raw materials from start tofinish of an industrial process. However, a main process stream may alsodenote a stream of raw materials other than those providing theinventive alkalinity, or it may denote loops and recirculations ofprocess fluids such as circulations of process water at various pointsof the industrial process. The industrial process may operate in acontinuous or batch type operation, although a continuous operation ispreferred.

The main process stream according to the invention is preferably anaqueous stream in an industrial process where raw material(s) flow fromstart to finish. The main process stream may also be a stream whichleads to or is separated from the raw material flow. It may be e.g. anaqueous pulp suspension in a fiber plant or in a paper machine or awater stream leading to and/or separated from said suspension. The mainprocess stream may also be a pulp suspension after a bleaching stage, ina stock preparation, in a long circulation or in a short circulation. Itmay comprise wash water recirculations or fresh water lines. The mainprocess stream may also be a stream in a water or wastewater treatmentprocess.

The terms “stock preparation”, “long circulation” and “shortcirculation” in the present description have the meanings defined inPapermaking Science and Technology: Book 8, Papermaking Part 1, StockPreparation and Wet End, ed. Hannu Paulapuro, Fapet Oy, 2000 p. 125.

A basic substance according to the invention is a substance which actsas a base in the surroundings of the process in question. The basicsubstance may be added in a solid, liquid or gaseous form and it may bedissolved or diluted in the apparatus. A preferred basic substance is analkali metal hydroxide, alkali metal carbonate, alkali metalbicarbonate, alkali metal phospate, alkali metal biphosphate and/oralkali metal phosphite. A preferred basic substance is liquid or solidsodium hydroxide, sodium carbonate, sodium bicarbonate, sodiumphosphate, calcium hydroxide, potassium hydroxide or green liquor orwhite liquor of a wood digestion process.

An acidic substance is a substance which acts as an acid in thesurroundings of the process in question. The acidic substance may alsobe a gas providing an acid in an aqueous medium. This means that it is agaseous substance which forms an acid in aqueous medium, e.g. gaseouscarbon dioxide or sulfur dioxide. Carbon dioxide is a gas, which easilydissolves under alkaline conditions, e.g. in water or in a pulpsuspension forming carbonic acid and/or bicarbonate ions according tothe reaction: CO₂+H₂O<=>H₂CO₃<=>H⁺+HCO₃ ⁻. At a high pH, especiallygreater than 10, the predominant reaction is CO₂+2OH⁻<=>CO₃ ²⁻+H₂O.

An aqueous solution of an acidic substance can also be called an aqueousacid. Thus carbon dioxide is an acidic substance which forms an aqueousacid. The acidic substance may also be an organic or inorganic acid,preferably selected from sulfurous acid, alun, phosphoric acid, carbonicacid, citric acid and hydrochloric acid.

In case sodium hydroxide or potassium hydroxide is used, a preferredacidic substance is carbon dioxide, sulfur dioxide, sulfurous acid,carbonic acid or phosphoric acid. The most preferred combination issodium hydroxide and carbon dioxide. Preferable combinations of thebasic and the acidic substance are e.g. a salt of carbonate or aphosphate and a strong acid, such as hydrochloric acid or alun.

A controlled aqueous solution of a basic and/or and acidic substancemeans a solution of the corresponding substance that has a specificconcentration. The solution may be prepared in advance separately from aconcentrated or solid substance and then fed into the apparatus.However, one advantage of the apparatus is that the controlled aqueoussolution can also be made automatically in the apparatus. For example incase NaOH is used as the basic substance the apparatus preferablycomprises means for diluting NaOH which has an initial concentratione.g. of about 30% to provide a NaOH solution having a concentration of1–9 w-%, preferably about 6%.

The water used in the apparatus may comprise raw water or waterseparated from said main process stream of the industrial process. Thewater may be derived e.g. from a water stream leading to and/orseparated from a pulp suspension or papermaking stock. The water mayalso comprise diluted pulp suspension separated from said main process.The water may also be drinking water to be treated in a water treatmentprocess. The alkalinity of drinking water is preferably raised justbefore the water is supplied to the water distribution system. Thealkalinity of wastewater is preferably controlled during the wastewatertreatment.

The present invention provides an aqueous buffering mixture for use inan industrial process. The mixture prepared in the apparatus in thereaction between the basic substance and the acidic substance whichsubstances in combination are capable of forming buffering ionsinfluencing the alkalinity of an aqueous medium. The resulting aqueousbuffering mixture has a predefined alkalinity which depends on thealkalinity needed in the industrial process in question. The aqueousbuffering mixture is capable of providing an elevated alkalinity in theindustrial process when it is supplied thereto.

The buffering mixture of the invention is formed from the basicsubstance, acidic substance and the water which are introduced in acontrolled way into the apparatus. The basic substance and the acidicsubstance react in the water in the apparatus providing a predefinedalkalinity in the resulting aqueous buffering mixture. The control meansof the apparatus assure that the buffering mixture has a desiredalkalinity and pH. The concentration of the basic substance ispreferably controlled and monitored by measuring e.g. the density of thebasic substance. The amount of the acidic substance fed into the reactoris then adjusted to obtain a desired alkalinity and pH in the resultingbuffering mixture. The resulting aqueous buffering mixture is suppliedto the main process stream of the industrial process for controlling thealkalinity and the pH of said industrial process.

The pH in the reactor is preferably also controlled and monitored. ThepH of the resulting aqueous buffering mixture is controlled according tothe needs of the industrial process. In a paper or pulp making process apreferred pH may be e.g. about 6 to 10. Also the carbonate ionconcentration, bicarbonate ion concentration and/or total alkalinity orM-alkalinity may be monitored in the apparatus. Thus the alkalinity ofthe resulting aqueous buffering mixture is adjusted to a specificpredetermined value in the apparatus. The resulting aqueous bufferingmixture is supplied into the industrial process to control thealkalinity of the main process stream.

The temperature and the pressure in the reactor are preferablycontrolled and monitored in the apparatus in order to keep theconditions suitable for the reactions between the water and the basicand the acidic substance. The temperature is preferably controlledbecause the reactions may be highly exothermic. Too high a temperaturemay cause an increase in the pressure or even an explosion.

Mixing of the substances is preferably provided to ensure proper andsafe reacting of the substances. In a multi-stage reaction where acid orbase is gradually added, the previous reaction should be completedbefore any new substance is added. The apparatus may comprise means forquick closing of the inlets in case e.g. the temperature or the pressurebecomes too high in the reactor.

In some cases the water used in the reaction contains free calcium ions.Free calcium ions may cause problems in many processes e.g. when theyprecipitate with other compounds in the process. Calcium ions may leade.g. to unwanted scaling on the walls. In a preferred embodiment of theinvention it is possible to keep calcium carbonate from precipitating onthe walls of the reactor by cooling the walls of said reactor. Anyprecipitated calcium carbonate will thus remain in the solution and willbe fed as solid particles to the main process with the buffering mixtureor, alternatively it may be removed from the mixture. In case thehardness of water is high the water can be treated in order to reducethe hardness and the amount of calcium ions. The water can be treatede.g. magnetically or by precipitation. Such methods are availablecommercially.

In a papermaking process it might be advantageous that any calcium ionsprecipitate in the apparatus. Especially when the basic substancecomprises NaOH which is contacted with calcium ions in the water, thesubsequent feeding of CO₂ as the acidic substance will precipitate thecalcium ions as calcium carbonate. The precipitated calcium carbonatewill be fed to the papermaking process and will attach to the fibers inthe pulp suspension and be removed from the circulating water.

In a preferred embodiment of the invention the apparatus is used forcontrolling the alkalinity using sodium hydroxide as the basic substanceand carbon dioxide as the acidic substance. The sodium hydroxide fedinto the apparatus can also be concentrated The sodium hydroxide may atneed be diluted in the apparatus prior to introduction into saidreactor. A preferred concentration after dilution is about 1 to 9%, morepreferably NaOH is diluted to a concentration of about 6%. The dilutionof the sodium hydroxide may be performed in the inlet end of the reactorat a point prior to the introduction of the carbon dioxide, or thesodium hydroxide may be separately pre-diluted

In a preferred embodiment carbon dioxide is introduced in at least twoseparate steps, preferably in 2 to 10 separate steps. In case a tubereactor is used carbon dioxide is preferably introduced in a tubereactor gradually e.g. through a perforated tube located inside the tubereactor.

The invention is now illustrated in a few examples which do not limitthe invention in any way. A person skilled in the art will be able touse the apparatus in many different ways.

EXAMPLE 1

An apparatus according to the present invention was tested in a papermill having the process characteristics shown in FIG. 2. The apparatus26 of the present invention was a tube reactor located outside the mainprocess 20 of the paper mill as disclosed in FIG. 2. The apparatus 26was connected to the controlling and monitoring system of the papermill.

The apparatus 26 was used for preparing a buffering mixture containingsodium and bicarbonate ions by absorbing gaseous carbon dioxide 28 intoa diluted aqueous solution 29 of sodium hydroxide (about 2%). The pH ofthe mixture was about 8.0. The alkalinity of the mixture was about 500mmol CaCO₃/l.

A pulp suspension 20 comprising a birch line and having a pH of 5.0 waspumped from a pulp mill storage tower 21 to a filter 22 in the papermill. The consistency of the pulp suspension after the filter was about30%. The buffering mixture prepared in the apparatus 26 was suppliedinto the pulp suspension at a dilution screw 23 after the filter 22 inan amount of 6 kg/1000 kg mass (dry weight basis). The pulp suspensionwas diluted at the same time to a consistency of about 10% using water30 in addition to the buffering mixture. The resulting pulp suspensionwas pumped through a drop pipe 24 to a paper mill storage tower 25. ThepH in the tower was about 7.2 and the alkalinity was 7 to 8 mmolCaCO₃/l.

The pulp was picked up from the storage tower 25 to the papermakingprocess 27 with a CaCO₃-containing white water 31 from the subsequentpaper machine which operated at a pH of 8.0. Since the pH of the pulpsuspension had been raised by the buffering mixture to 7.5 and becauseof the increased alkalinity of the pulp suspension, the CaCO₃ in thewhite water remained in solid form in the resulting diluted suspension.In case no buffering mixture had been used, the CaCO₃ would havedissolved in consequence of the pH variation which would have takenplace when the pulp suspension at pH 5.0 met the white water at pH 8.0.As a consequence of the feeding of the buffering mixture to the processat the screw 23, the whole of the subsequent papermaking process becamestabilized and the M-alkalinity of the white water was raised from 2.6mmol CaCO₃/l to 4.5 mmol CaCO₃/l. The hardness of the white waterdecreased about 30% from 65 Ca mg/l to 28 Ca mg/l.

The above mentioned procedure was also applied to a pine line in thesame paper mill. The procedure was the same expect that the bufferingmixture was supplied to the drop pipe 24 distribution ring. Similaradvantageous results were obtained as when used in the birch line.

EXAMPLE 2

An apparatus according to the invention was tested in preventing pHchanges in a broke storage tower caused by the organic acids formed inconsequence of the microbiological activity.

Broke from several places in a paper making process were collected in astorage tower. An apparatus according to the invention was connected toa recycling pipe used in the storage tower to help homogenize the brokein the tower. A buffering solution having a pH 7.5 was intermittentlysupplied in an amount of 5 kg NaHCO₃/1000 kg mass (dry weight basis) tothe recycling tower. The pH of the broke remained steady and no decreasein pH was observed despite microbiological activity.

EXAMPLE 3

Water in a water supply system had a low alkalinity (0.2 mmol CaCO₃/l).The low alkalinity caused pH fluctuations in the supply system and this,in turn, caused corrosion problems in the pipes.

The alkalinity of the water was raised with the apparatus according tothe invention using NaOH and CO₂ to a value of 1.0 mmol CaCO₃/l.

The pH of the buffered water remained constant at 7.0 and no corrosionproblems occurred in the pipes.

It is evident that the invention may be varied in a great number of wayswhich are obvious to those skilled in the art without deviating from thescope of the claims.

1. An apparatus for controlling the alkalinity, pH or hardness of a mainaqueous process stream of an industrial process, the apparatus beingadapted for being located outside the main process stream of saidindustrial process, said apparatus including inlet means (2, 3, 4) for abasic substance, an acidic substance and water, control means (6) forsaid inlet means (2, 3, 4), for controlling the feeding of the basicsubstance, the acidic substance and the water, a reactor (1) forreacting said basic substance, said acidic substance and said water forproviding a predefined alkalinity and/or pH in a resulting aqueousbuffering mixture, and outlet means (9) connected during use to the mainprocess stream for supplying said buffering mixture to said main processstream for controlling said industrial process.
 2. An apparatusaccording to claim 1, wherein said industrial process is a process formaking paper or pulp or a water treatment process for fresh water orwaste water.
 3. An apparatus according to claim 1, wherein saidapparatus is connected to a control and monitoring system of saidindustrial process or said main process stream.
 4. An apparatusaccording to claim 1, wherein said inlet means (2) for a basic substancecomprises an inlet for solid or liquid base selected from the groupconsisting of sodium hydroxide, sodium carbonate, sodium bicarbonate,sodium phosphate, calcium hydroxide, potassium hydroxide and greenliquor or white liquor; and said inlet means (3) for an acidic substancecomprise an inlet for gaseous carbon dioxide or sulfur dioxide or for aliquid organic or inorganic acid, selected from the group consisting ofsulfurous acid, alum, phosphoric acid, carbonic acid, and hydrochloricacid.
 5. An apparatus according to claim 1, wherein said inlet means (2,3) include means for diluting and/or dissolving the basic and/or acidicsubstance in water to provide a controlled aqueous solution of saidbasic and/or acidic substance.
 6. An apparatus according to claim 1,wherein said inlet means (3) for an acidic substance comprise an inlet(5) for liquid acid or for a gas which is capable of forming an acid inan aqueous medium.
 7. An apparatus according to claim 6, wherein saidinlet means (2) for a basic substance comprise an inlet for dilutedsodium hydroxide and said inlet means (3) for an acidic substancecomprise an inlet (5, 7) for gaseous carbon dioxide.
 8. An apparatusaccording to claim 1, wherein said inlet means for water (4) comprise aninlet for water or a dilute pulp suspension separated from said mainprocess stream.
 9. An apparatus according to claim 1, wherein said inletfor water is combined with said inlet (2, 3) for a basic substanceand/or said inlet for an acidic substance.
 10. An apparatus according toclaim 1, wherein said inlet control means (6) comprise means formonitoring and adjusting the flow and/or concentration of said basic andacidic substances and the flow of the water in order to provide saidpredefined alkalinity.
 11. An apparatus according to claim 1, whereinsaid apparatus comprises a continuous reactor or a batch reactor.
 12. Anapparatus according to claim 1, wherein said reactor comprises a tubereactor (1) having a length which is sufficient to allow the reactionbetween said substances to react in a controlled manner.
 13. Anapparatus according to claim 12, wherein said tube reactor (1) comprisesseveral inlets for said basic and/or acidic substance and/or water. 14.An apparatus according to claim 12, wherein said tube reactor (1)includes two or more inlets (7) for said acidic substance said inletsbeing spaced apart along the tube reactor.
 15. An apparatus according toclaim 1, wherein said reactor comprises control means (8) forcontrolling a parameter selected from temperature, pressure, flow amountand velocity, mixing and pH.
 16. An apparatus according to claim 15,wherein said reactor control means (8) comprise means for cooling thewalls of said reactor in order to keep calcium carbonate fromprecipitating on the walls.
 17. An apparatus according to claim 1,wherein said reactor comprises control means (8) for removal ofprecipitated matter.
 18. An apparatus according to claim 1, wherein saidapparatus is adapted for being located outside and connected to a stockpreparation, a water circulation line, a short circulation, a filterbetween a pulp mill and a paper mill, a broke storage tank and/or aclear filtrate line in a papermaking process.
 19. An apparatus accordingto claim 1 wherein said apparatus comprises a tank for said bufferingmixture.
 20. The apparatus of claim 1, further comprising a tank forholding said buffering mixture, said tank being located optionally upstream of said outlet means (9).
 21. A method for controlling thealkalinity, pH or hardness of an industrial process, said methodcomprising providing a basic and an acidic substance which substances incombination are capable of forming buffering ions which influence thealkalinity of an aqueous medium; introducing controlled feeds of saidbasic and said acidic substances as well as water into a reactor of anapparatus located outside a main process stream of said industrialprocess; causing said basic substance and said acidic substance to reactin said water to provide a predefined alkalinity and/or pH in theresulting aqueous buffering mixture; supplying said aqueous bufferingmixture to said main process stream for controlling the industrialprocess.
 22. A method according claim 21, wherein said industrialprocess is a process for making paper or pulp or a water or a wastewater treatment process.
 23. A method according claim 21, wherein saidmain process stream comprises an aqueous pulp suspension in apapermaking process or a water stream leading to and/or separated fromsaid suspension.
 24. A method according claim 21, wherein said basicsubstance comprises an alkali metal hydroxide, an alkali metalcarbonate, an alkali metal bicarbonate, an alkali metal phosphate, analkali metal biphosphate and/or an alkali metal phosphite and/or greenliquor or white liquor, and that said acidic substance comprises gaseouscarbon dioxide or sulfur dioxide or a liquid organic or inorganic acidselected from the group consisting of sulfurous acid, alum, phosphoricacid, carbonic acid and hydrochloric acid.
 25. A method according claim21, wherein said carbon dioxide is introduced in a tube reactor in atleast two separate steps.
 26. A method according claim 21, wherein saidwater comprises process water separated from said main process streamand/or water leading to said main process stream.
 27. A method accordingclaim 21, wherein said water contains calcium ions, and the walls ofsaid reactor are cooled in order to keep calcium carbonate fromprecipitating on the walls, and in that any precipitated matter isoptionally removed from the buffering mixture prior to supplying thesame to said main process stream.
 28. A method according claim 21,wherein the temperature, pressure, mixing and/or pH of said reactionand/or the flow of said substances is monitored.
 29. A method accordingclaim 21, wherein the pH of said aqueous buffering mixture is controlledto about 6 to
 10. 30. A method according to claim 21, wherein said wateris treated in order to reduce the hardness of water.
 31. A methodaccording to claim 30, wherein said water is treated magnetically or byprecipitation.
 32. A method according to claim 22, wherein saidindustrial process is controlled so as to retain calcium carbonate insolid form.
 33. The method of claim 25 wherein said carbondioxide isintroduced into the tube reactor in more than two steps.
 34. The methodof claim 25 wherein the carbondioxide is introduced into the tubereactor through at least three to ten steps.
 35. The method of claim 21wherein said main process stream contains calcium carbonate, and theindustrial process is controlled so as to retain calcium carbonate insolid form.
 36. A method for controlling the alkalinity, pH and/orhardness of an industrial process, said method comprising providing abasic substance and an acidic substance, which substances in combinationare capable of forming buffering ions which influence the alkalinity ofan aqueous medium; introducing controlled feeds of said basic substanceand said acidic substance as well as water into a reactor of anapparatus located outside a main process stream of said industrialprocess; causing said basic substance and said acidic substance to reactin said water to provide a predefined alkalinity and/or pH in theresulting aqueous buffering mixture; and supplying said aqueousbuffering mixture to said main process stream for controlling theindustrial process; wherein said basic substance comprises sodiumhydroxide and said acidic substance comprises carbon dioxide.